Heretofore, there has been a problem that, during walk of a walking robot having a plurality of movable legs, a leg link mechanism or precision apparatuses, such as sensors, included in the leg link mechanism are apt to be broken due to a touch-down impact caused when the leg link mechanism and the external environment, including the ground and a floor, collide with each other. In order to prevent this problem, some walking robots include, in the foot of the extremities of the movable legs of the robot, an impact absorbing mechanism for absorbing the touch-down impact, for example, which uses rubber bushes with low rigidity and the like.
Some of those walking robots further include a 6-axis force sensor. This sensor is used for control of walk by the walking robots having movable legs, in particular, for compliance control of the foot joints with regard to control of bipedal walking. In the foot of the extremity of each of the movable legs, the sensor is installed between a lower base plate and an upper base plate. The lower base plate has a surface to contact the ground, a floor and the like, and is an equivalent to the foot sole. The upper base plate is joined to the foot joint, and supports the upper structure of the robot including the rest part of the movable leg.
Furthermore, with regard to a compliance control of the foot joints in control of walk of a bipedal walking robot, the 6-axis force sensor measures a reaction force from the lower base plate, which is caused due to a contact of the lower base plate mainly with the ground and the like, as force components respectively in the yaw axis direction (perpendicular direction), the roll axis direction (antero-posterior direction) and the pitch axis direction (left-right direction), as well as moment components respectively about the axes. On the basis of these parameters, a CPU (central processing unit) included in the main body of the robot performs calculations. Thereby, each of the joints in its movable legs is controlled.
At this point, with regard to the foot mechanism provided with elastic members, such as the rubber bushes, constituting the impact absorbing mechanism, the force components and the moments about the axes to be measured by the 6-axis force sensor have the respective deviations due to elastic displacements of the elastic members respectively with regard to the yaw-axis, the roll-axis and the pitch-axis. When the walking of the robot is controlled, this complicates the calculation by the CPU included in the main body of the robot.
If the deviations of the force components in the respective axis directions and the deviations of the moments about the respective axes were kept constant, this can make it simple to control the walking of the robot. To this end, with regard to the elastic displacements in the elastic members which are interposed between the lower base plate and the upper base plate in each of the foot mechanisms for the purpose of impact absorption, it is preferable that the respective rotational spring constants concerning displacements relatively of the base plates be kept constant, and that displacement to maintain the relative positional relationship between the base plates be made isotropic.
More specifically, it is preferable that a walking robot be configured to restrict displacements (deviations) in the axis directions respectively of the lower base plate and the upper base plate, which are inappropriate for controlling of the walking of the robot, by the following measures. In order that elastic displacement with low rigidity due to the impact absorbing mechanism including the rubber bushes and the like may absorb a reaction force from the lower base plate and the load from the upper base plate, including the dead weight of the robot, first, elastic displacement with low rigidity concerning the perpendicular direction (yaw-axis direction) is allowed. Second, concurrently, the elastic displacement has a high rigidity concerning the axis directions orthogonal to the perpendicular direction.
With regard to the walking robot having movable legs, however, when the lower base plate of each of the legs touches down to a slope, the ground in a rough terrain condition or the like, the elastic members, such as the rubber bushes, constituting the impact absorbing mechanism cause disparate elastic displacements which respectively vary in displacement amount with regard to each of the axis directions. For this reason, rotational spring constants respectively of the elastic displacements can not be constant. Accordingly, displacements for maintaining the relative positional relationships between the base plates are hindered from being isotropic.
In addition, while the robot having movable legs is walking, in particular while a bipedal walking robot is walking, when a free leg (a leg in motion, which is not in contact with the ground) is swung forward, this causes a torque about the yaw axis (a moment of rotation) in a supporting leg (a leg being in contact with the ground, and supporting the load including the dead weight of the robot). Accordingly, a large spinning force acts about the yaw axis with the supporting leg working as a center of rotation. This causes the elastic members, such as the rubber bushes, constituting the impact absorbing mechanism to respectively make elastic displacements due to the distortion chiefly about the yaw axis. Concurrently, the elastic displacements respectively about the roll axis (the axis in the antero-posterior direction) and the pitch axis (the axis in the left-right direction), both of which are orthogonal to the yaw axis direction, become so disparate that their displacement amounts differ from each other. As a result, the displacements for maintaining the relative positional relationship between the base plates cannot be isotropic. This complicates the control of the walking of the robot.
In order to restrict such disparate elastic displacements respectively of the elastic members, whose respective displacement amounts vary from one to another, and in order to accordingly make isotropic the relative displacement between the upper base plate and the lower base plate which is an equivalent to a foot sole, it has been essential that a conventional robot be provided with a guide mechanism using rigid members such as plates, which allow the elastic members to make the respective elastic displacements in the yaw axis direction, and which have high rigidity concerning the axis directions orthogonal to the yaw axis direction. This restricts displacements (deviations) in the axis directions between the lower base plate and the upper base plate, which are inappropriate for controlling of the walking of the robot (see Japanese Patent Laid-open No. Hei. 11-033941, Paragraph [0029], and FIG. 1, for example).
However, if the guide mechanism using the rigid members in this manner were installed in the robot, this brings about the following problem. First, the installation increases the weight of the foot mechanism, accordingly increasing the inertial moment, which needs to be suppressed in each of the movable legs of the walking robot. In addition, this installation causes each of the members with high rigidity and a corresponding elastic member to contact each other, accordingly causing frictional resistance. As a result, this frictional resistance, as disturbance, acts on the control of the walking of the robot.
Furthermore, the additional installation of the guide mechanism using the rigid members brings about another problem. The additional installation complicates the foot mechanism. Depending on conditions of the walking posture of the robot, the rigid members interfere physically with the 6-axis force sensor in conjunction with displacement of the elastic members such as the rubber bushes. Accordingly, this breaks the force sensor.
Moreover, with regard to the compliance control, while the 6-axis force sensor measures a reaction force from the lower base plate to be caused due to the lower base plate's contact chiefly with the ground and the like, the impact absorbing mechanism, such as the rubber bushes, for absorbing an impact caused due to the lower base plate's contact with the ground and the like, absorbs the touch-down impact. For this reason, the impact absorbing mechanism is preferable in protecting the robot's joint structures including the sensor devices such as the 6-axis force sensors, and the leg link mechanism. However, the elastic displacements with low rigidity, which are properties of the impact absorbing mechanism, causes vibrations. Accordingly, the vibrations remain during the period of vibration damping. This inadequately vibrates the 6-axis force sensor. As a consequence, the vibration acts, as disturbance, on the control of the walking of the robot.